The present invention relates to systems for controlling devices on a vehicle, and more particularly to wireless control systems.
Automobiles, trucks and trailers have numerous devices, such as lights and actuators, which are electrically operated. For example tail lights, brake lights, left and right turn signal indicators, and back-up lights are all mounted at the rear of a typical automobile. Each type of light requires that a separate power wire be run from the dashboard to control the light's operation. Similar groups of lights are mounted at the front of the vehicle which require another set of electrical wires. In addition, different actuators are located in the engine compartment and also receive control signals. In all, numerous bundles of wires run throughout the motor vehicle in order to control and operate the various devices.
It is desirable to merely run a pair of wires that form a power bus throughout the vehicle and provide a wireless mechanism for sending control signals to the individual devices. Such a mechanism must provide a technique by which several sets of controllers and devices can communicate simultaneously. In addition, wireless communication within a particular vehicle can not be interfered with by similar communications occurring in a nearby vehicle. Thus a robust communication protocol must be utilized.
Bidirectional radio frequency communication has been used for some time in cordless telephones. The term "cordless telephone" as used in the telecommunication industry, means a telephone comprising a base station and a hand-held transceiver unit. The base station is connected by wires to a terrestrial telephone line serving the owner's premises. A hand-held transceiver carried by the user communicates by radio frequency signals with the single base station that is up to approximately 300 meters away.
The Digital Enhanced Cordless Telecommunications (DECT) protocol was developed in the mid-1980's as a pan-European standard for cordless telephones and has been adapted for use outside the European Union. The DECT standard protocol has been used for simultaneous bidirectional communication between a base station and a hand-held transceiver of cordless telephones. This standard utilizes ten frequencies for communication. The exchange of signals over each frequency is divided into repetitive frames 10, each being ten milliseconds in duration and subdivided into twenty-four time slots, as shown in FIG. 1. The twelve time slots in the first half 14 of each frame are used for communication from a hand-held transceiver to the associated base station, while the twelve time slots in the second frame half 16 are used for communication from the base station and the hand-held transceiver. It should be noted that different regions of the world have implemented the DECT protocol is slightly different manners. For example, in some regions the frequencies and the number of time slots in each message frame may differ.
When a user desires to use activates the cordless telephone to make an outgoing call, the hand-held transceiver searches for a frequency that has a matching time slots in each frame half which are not being used by another cordless telephone system. This is accomplished by the hand-held transceiver listening for digital signals being sent in each time slot of the frame at each of the assigned frequencies. When a vacant pair of time slots, such as 18 and 19, is found, the hand-held transceiver sends a message initiation signal on the selected frequency during time slot 18 in the first half of a message frame.
While the hand-held transceiver is performing these functions, the base station is scanning the ten frequencies and listening during each of the twelve time slots in the first half 14 of the message frames at each frequency. When the base station hears a message initiation signal that is addressed to it, i.e. containing the proper identification data, the base station sends a response to the transceiver in the associated time slot 19 in the second half 16 of a frame at the same frequency and bidirectional communication is established. A reverse procedure occurs when the base station receives an incoming call via the terrestrial telephone line.